Overheating and smoking

I have a 1993 626, 2.0 4-cylinder standard with 188,888 miles. Recently it started to overheat when going up large hills, but only after being driven for hours. It also over heats when traveling at high speeds for long periods of time, and the a/c makes this problem worse. It's never gotten into the red or boiled over. The needle will stay steady for hours and then suddenly jump to just below the red. At that point I immediately slow down and the car's temp will drop very quickly. I've never kept pushing it to see if it will boil over, but the needle seems to stay steady just below the red. I have no ideas on this as it only happens after being driven for long periods of time and only when the engine is taxed.

Occasionally when going over passes the "malfunction indicator light" will come on. If I stop, turn the car off and back on the light goes off, or if I slow down the light will go off and stay off the rest of the drive.

Yesterday while driving around town the car started blowing a lot of smoke out the tail pipe and smelled like burning oil.

I'm totally stumped as to what is going on. I drive long distances a lot and usually am not close to towns. Please help!